Obesity is a rapidly growing health problem in the US for which neither effective treatment nor preventive measures are currently available. It is now widely accepted that environmental factors that encourage eating and discourage physical activity are important contributors to the problem, and it has been suggested that environmental interventions may be needed to achieve reductions in population obesity. The proposed study will assess the efficacy of a multi-component environmental intervention in preventing weight gain among working adults. Eight worksites will be randomized to either an intervention or a no-treatment control group. Intervention will be comprised of 1) changing the availability, portion sizes, and prices of foods and beverages sold to employees in their worksites in ways that encourage healthier food choices; 2) increasing the availability of physical activity opportunities at the worksite by implementing walking programs and increasing stairwell access and attractiveness; 3) placing scales in the work environment to encourage body weight monitoring and to enable workers to set goals for their weight; and 4) to provide educational materials to all employees to make them aware of the environmental interventions and of behavioral practices likely to be effective in preventing weight gain. Body weight, eating behavior, and physical activity will be measured at baseline and two years in a cohort of employees from each worksite. Aggregate measures of behavior will also be obtained through direct observation and management records. It is hypothesized that employees in intervention worksites will decrease energy intake, increase energy expenditure, and reduce weight gain compared to those in comparison sites over two years. It is also anticipated that the effects of the intervention on behavior and weight will be related to degree of exposure to intervention activities.